Saturnalia falls at the time when non-Romans are celebrating Christmas, Hanukkah, Solstice and/or Kwanzaa. In Nova Roma, individual Citizens have chosen different approaches to the challenge of celebrating in the spirit of Rome without cutting themselves off from the culture in which they live.
For a compilation of Saturnalia celebrations reported by Nova Romans, visit Saturnalia practices of Nova Romans. For a detailed historically acurate ritual visit Saturnalia - A Winter Solstice Ritual.
Here's some History:
Saturnalia festivities began with ritual and sacrifices in the Temple of Saturn. The statue of the god was hollow and filled with olive oil, as a symbol of his agricultural functions. His feet were bound with woolen strips, that were unbound at Saturnalia.
After the rituals, the Senators (who had to be present) dismissed the crowd with the cry of "Io, Saturnalia!", a sign for the happy festivities of family parties and other private gatherings to begin. The traditional gifts were wax tapers and little dolls, although gifts of silver later became traditional.
The custom of the Lord of Misrule was appropriated and survived through to English Christmas traditions.
Many of the decorations involved greenery - swathes, garlands, wreaths, etc - being hung over doorways and windows, and ornamenting stairs. Ornaments in the trees included sun symbols, stars, and faces of the God Janus. Trees were not brought indoors (the Germans started that tradition), but decorated where they grew.
Food was also a primary decoration - gilded cakes in a variety of shapes were quite popular, and children and birds vied for the privilege of denuding the trees of their treats. The commonest shapes were fertility symbols, suns and moons and stars, baby shapes, and herd animal shapes (although, to be honest, it's hard to tell if some of those ancient cookie cutters are supposed to be goats or deer). I would imagine coins were also a popular decoration/gift.
People were just as likely to be ornamented as the trees. Wearing greenery and jewelry of a sacred nature was apparently common, based on descriptions, drawings, and the like from the era. Although the emphasis was on Saturn, Sol Invictus got a fair share of the revelry as well.
Gold, because the sun is yellow, is always a sure choice for a good Saturnalia decoration. For modern Saturnalia, those golden glass ball ornaments are ideal, as are gold sun faces, gold stars, and gilded anythings. Gilding nuts and pine cones and nestling them among the swags and wreaths of greenery would be a lovely way of acknowledging the ancient roots of this ceremony.
Indoor trees are not ancient Roman, but if you have plants growing indoors, decorating them would certainly be in the spirit of the holidays. If you just have to have the now-traditional indoor tree, try decorating it in gold ornaments with a solar theme. Swathe it in bright red or purple ribbons (2 colors quite in favor with the Romans, and looks great with the gold ornaments). Top the tree with a sun, rather than a star, for after all, this is a solar celebration.
The biggest part of Saturnalia was attitude more than decoration. Feasting, drunkenness, merrymaking, hopefully the conception of more children (or at least enjoying those activities which led to conception!), pranks, gift giving, role reversals (not true ones, only symbolic ones - slaves weren't really free to make a freedman's decisions and anything they did or decreed would reverse at the end of Saturnalia, children weren't really adults and could not enter into any binding contracts or make business deals, etc.) and so forth.
The role reversals seemed to be more for minor privileges - slaves and children got to be waited on for meals, and to lead the rituals, and to participate in the revelry as if they were their parents/masters. The parents/masters jokingly played the part of slaves and children by waiting on them and making rude and bawdy jokes at their expense. Sometimes, it descended into cruelty.
On a modern note - wild parties with lots of food and drink is good. Letting the children of the household lead the common rituals, and waiting on them (assuming you don't do so in everyday circumstances....) at mealtimes, and deferring to them in decisions on party ideas would work for role reversals.
No children in the house? Maybe you can borrow one for a day. We don't have slaves, but, for a nice touch of role reversal, we could purchase the services of a nanny or a housekeeper for the duration of Saturnalia. It would be a role reversal of sorts, for instead of being the slave of your home, someone else would be doing the chores and cooking and childcare while you got to party down!
Dancing and singing in the streets is now frowned upon, unless you can get a parade permit. A parade, if you could organize it, would be fun. Imagine - giant floats of the Gods tended by the priests and acolytes, musicians and dancers, contortionists to amaze and delight, acrobats and jugglers, all in honor of Saturn!
Source: Nova Roma